1. Field of the Invention
The present invention relates to an intake system for an internal combustion engine equipped with a supercharger.
2. Description of Related Art
Intake systems for use with internal combustion engines equipped with mechanical superchargers typically include intake pipes, individually provided for cylinders, for introducing air into the cylinders, throttle bodies with built-in throttle valves, and inter-coolers for cooling air downstream from the mechanical superchargers.
In such an intake system, the throttle body is conventionally disposed between an upstream intake pipe, including an air cleaner, and the mechanical supercharger, and is typically mounted on or supported by an engine as well as the mechanical supercharger. Internal combustion engines operate at fairly high temperatures, and temperature differences are caused locally. Consequently, if an internal combustion engine causes a relatively large local temperature difference between a portion of the intake system in which the throttle body is mounted and a portion in which the mechanical supercharger is mounted, the two portions are subjected to largely different thermal expansions. An undesirable external force, therefore, is applied to the throttle body and the mechanical supercharger. The mechanical supercharger, moreover, typically suffers from different modes of vibration. One such mode is transmitted from the engine through the valve body, and the other is a self-generating heavy vibration caused by operation of the mechanical supercharger at a high speed of rotation. Such an undesirable external force and the different modes of vibration decrease the durability and the operative life of the mechanical supercharger.
To eliminate the transmission of vibration of the engine to the mechanical supercharger through the throttle body, it was thought that the throttle body should be "cantilevered" by the mechanical supercharger. In such a structure, however, since the throttle body is made of metal and is fairly heavy, a decrease in durability of a connection between the throttle body and the mechanical supercharger and/or between the throttle body and part of the intake system upstream from the throttle body may occur.
In addition, noise is caused by the intake system, due to vibration of an air column generated in the intake pipe by the mechanical supercharger, as well as due to vibration of the throttle body. To eliminate a specific frequency of noise, it has been proposed to provide a resonator in this kind of intake system. Such an intake system is known from, for instance, Japanese Unexamined Patent Publication No. 59-218,356.
The intake system described in the publication mentioned above reduces vibration to some extent. However, it is still hard for such an intake system to eliminate or reduce vibration caused by and around the throttle valve.
Individual intake pipes typically merge into an air integration device, such as a surge tank. The air integration device is provided for stabilizing air flowing into the cylinders through the individual intake pipes. Intake air interference may possibly be caused in the integration device between adjoining individual intake pipes of cylinders which perform their intake strokes one after another. In order to eliminate intake air interference and, therefore, avoid a decrease in air charging efficiency, it is typical to provide one integration device for individual intake pipes of each of two groups of cylinders which do not fire one after another. This results in the provision of two intake systems. Such intake systems are known from, for instance, Japanese Unexamined Patent Publication No. 2-227,517.
The provision of two intake systems makes the whole intake system structure bulky. Additionally, if path lengths of the two intake systems are different, responsiveness is apparently different between the intake systems. This causes irregularity in air-fuel control.
When intake air is supercharged, it is adiabatically compressed, and its temperature rises. Supplying heated air into a combustion chamber is undesirable for a sufficient increase in air charging efficiency. For this reason, the intake system is typically equipped with an inter-cooler in an air passage downstream from the mechanical supercharger. Because the film coefficient of heat transfer of air is very small, the inter-cooler should have a fairly large heat transfer area for efficient air cooling. Such an inter-cooler has a high resistance to intake air and is bulky.
It has recently been attempted to provide various devices and functional elements in engine rooms in order to realize high performance vehicles. On the other hand, it has also been a recent tendency to provide as low an engine room hood as possible. This leads to both a small, tight engine room and a difficulty in arranging the inter-cooler in such a small engine room. If the inter-cooler is an air cooling type, it is even more difficult to arrange the inter-cooler so that it is well ventilated in the small engine room.
Certain vehicles have power trains in which engines and transmissions are connected in series in transverse directions of the vehicle bodies. In such a vehicle, it is advantageous, in order to increase the cooling performance of an inter-cooler, to arrange the inter-cooler in what is called a "dead space," formed above the transmission, in which many large devices and elements are not arranged and which is well ventilated. However, since the dead space is usually relatively small, it is difficult to install an inter-cooler, having a sufficiently large capacity and area of heat transmission, in the dead space. Otherwise, in vehicles in which a V-type engine is arranged in a lengthwise direction of the vehicle body, increasing the cooling performance of the inter-cooler is realized by arranging the inter-cooler in front of the V-type engine so as to place its broadest face so that it is directed forward. This arrangement is not applicable, however, to transverse power trains.